An electrical bandpass filter is a fundamental element used for selecting an electrical signal in a frequency passband while suppressing electrical signals in a frequency stopband of the filter. Microwave and millimeter-wave bandpass filters are often used in modern radio-frequency transceivers. Filters having low in-band insertion loss, high spectral selectivity, and a wide stopband are commonly required. As an example, in a typical ground terminal for communication with satellites in the Ka frequency band, a filter is required to suppress signals at transmission frequencies in a 29.5 GHz-30 GHz frequency range while conveying the signals at reception frequencies in a 19.2 GHz-21.2 GHz frequency range. An insertion loss of less than 1 dB and a stopband suppression level of at least 45 dB are desired to select the signal while avoiding self-jamming effects during simultaneous reception and transmission of electromagnetic signals by the ground terminal.
Microwave bandpass filters can be implemented as bulk waveguide structures. These are relatively heavy, bulky, and expensive; due to their size and weight, integration of bulk waveguide filters with planar components and electronic circuits can be a challenging task.
Substrate integrated waveguides (SIWs) are waveguide structures formed in a substrate of an electronic circuit. SIWs allow easy integration of planar circuits on a single substrate using a standard printed circuit board (PCB) or low-temperature co-fired ceramic (LTCC) process, or any other process of planar circuit fabrication. By using SIWs in an electronic circuit, the interconnection loss between components can be reduced. The size and the weight of the entire circuit can also be reduced.
SIW filters are known in the art. They offer a low-cost, low mass and compact size alternative to conventional waveguide filters, while maintaining high performance. Although various techniques have been implemented to improve the stopband performance of conventional rectangular waveguide filters, these techniques often utilize E-plane discontinuities that are difficult to realize for SIW filters implemented on a single-layer substrate. The SIW filters of the prior art have often been limited to resonant structures based on physical coupling elements to achieve a pre-selected spectral shape of the filter response function and/or high levels of stopband suppression. For example, a SIW filter designed to block an electromagnetic signal at a frequency f0 has a slit in the top or bottom conducting layer to provide an attenuation pole at the frequency f0.
Transmission zeros (TZs) in the insertion loss response of a microwave filter can be used to improve the spectral selectivity and the stopband attenuation of the filter. To generate the TZs, an “extracted pole” technique can be implemented to construct so called “bandstop” resonators. Alternatively, electrical couplings can be introduced between non-adjacent resonators, wherein the TZs are generated due to a phenomenon of multipath interference of electromagnetic waves propagating inside the resonators. However, such filters are usually constructed using conventional waveguide technology, which tends to use bulky and complex filter structures. Furthermore, the TZs implemented using these prior-art methods cannot be far away from the desired passband due to the limitation of the physical structure of a prior-art waveguide filter.
The present invention overcomes the above stated problems of the prior art. It provides a low-cost, high-performance SIW filter that is easy to integrate with planar circuits. Advantageously, the spectral shape of the SIW filter of the present invention can be adapted to provide a high level of attenuation away from a desired passband. Furthermore, SIW filters can offer a significant improvement in passive intermodulation performance over conventional filters.